The present invention relates to a gas compressor and, more particularly, to such a device capable of miniaturization and requiring relatively low electrical power input.
Within the past decade, a variety of new super-conducting cryoelectronic devices have been developed based upon the Josephson effect. These devices include, for example, extremely sensitive magnetometers, gradiometers, bolometers, voltage standards, current comparators, rf attenuators and logic elements. See, e.g., IEEE Trans. On Magnetics, Vol. 17, No. 1, Jan., 1981, Sessions BC, CC, SC, HC, IC. These devices typically operate at temperatures below about 22.degree. K. (i.e., 22 degrees absolute), and the power dissipated by such devices is characteristically on the order of microwatts.
Several methods are available for obtaining the cryogenic temperatures required for these devices. The simplest approach is to use liquid helium, but this method requires elaborate Dewars, is expensive and cumbersome, and requires an available supply of liquid helium. More convenient methods include the use of closed-cycle mechanical refrigerators, which are generally well-known in the art. The two most familiar of these refrigerators are the Gifford-McMahon (modified Stirling) cycle, and the Joule-Thompson expansion cycle, discussed, for example, in Barron, Cryogenic Systems (McGraw-Hill, Inc., 1966). A typical Gifford-McMahon refrigerator has two stages, operates at 200 psig., and delivers approximately one watt of useful refrigeration at about 10 to 15.degree. K. A Joule-Thompson expansion cycle is commonly staged onto a Gifford-McMahon refrigerator, utilizes a 300 to 1/2 psig expansion, and delivers approximately three watts of useful refrigeration at 4.2.degree. K.
It will be recognized from the above that there is a great mismatch between the refrigeration requirements of the cryoelectronic devices, typically on the order of microwatts, and the refrigeration capacity of known mechanical refrigerators, typically on the order of watts.
A recent approach to matching these power considerations involves the microminiaturization of refrigeration systems using planar photoresist technology similar to that used in the semi-conductor industry. See e.g., NBS Special Publication 508, 75-80 (U.S. Dept. of Commerce, April, 1978). Although the Stirling, Gifford-McMahon, and Joule-Thompson systems all lend themselves to microminiaturization, the Joule-Thompson system appears most practical due to the absence of moving parts. Prototypes for such systems have been discussed in the prior art, designed to deliver about 20 milliwatts of useful refrigeration below 20.degree. K.
These micro-refrigerators, while bringing the device-refrigerator power considerations into commensuration, have yet to overcome a major practicality hurdle. In particular, a compressor suitable for driving such a refrigerator for a extended period of time is not presently known.
Suggested compressors have typically involved small gas cylinders or adsorption-desorption pumps. Gas cylinders, of course, have only limited lifetimes. Adsorption-desorption pumps operate on the principle that certain solids, such as zeolites or metal hydrides, selectively adsorb certain gases at a first temperature and pressure, and desorb them at a second, higher temperature and pressure. Therefore, by thermally cycling such a solid with appropriate valving, gas compression is achieved. These pumps are disadvantageous in that long cycle times are involved, typically on the order of 30 minutes, due to slow adsorption and heat-transfer rates. Further, the overall compression efficiency of such pumps is low.
What is needed, therefore, is a gas compressor ideally matched to the requirements of the micro-refrigerators described above. Such a compressor should be of small size, commensurate with the small size of the microminiature refrigerators. Further, the compressor should have relatively modest electric power requirements, and should be capable of supplying sufficiently large gas flow rates. Moreover, the compressor should be applicable to any gas.